Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally<1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a mask pattern is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
Although specific reference may be made in this text to the use of lithographic apparatus and masks in the manufacture of ICs, it should be explicitly understood that such apparatus and masks have many other possible applications. For example, they may be used in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “reticle”, “wafer” or “die” in this text should be considered as being replaced by the more general terms “mask”, “substrate” and “target portion”, respectively.
In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range 5-20 nm).
A lithographic mask typically may contain opaque and transparent regions formed in a predetermined pattern. The exposure radiation exposes the mask pattern onto a layer of resist formed on the substrate. The resist is then developed so as to remove either the exposed portions of resist for a positive resist or the unexposed portions of resist for a negative resist. This forms a resist pattern on the substrate. A mask typically may comprise a transparent plate (e.g. of fused slicia) having opaque (chrome) elements on the plate used to define a pattern. A radiation source illuminates the mask according to well-known methods. The radiation traversing the mask and the projection optics of the lithographic apparatus forms a diffraction-limited latent image of the mask features on the photoresist. This can then be used in subsequent fabrication processes, such as deposition, etching, or ion implantation processes, to form integrated circuits and other devices having very small features.
As semiconductor manufacturing advances to ultra-large scale integration (ULSI), the devices on semiconductor wafers shrink to sub-micron dimension and the circuit density increases to several million transistors per die. In order to accomplish this high device packing density, smaller and smaller feature sizes are required. This may include the width and spacing of interconnecting lines and the surface geometry such as corners and edges of various features.
As the nominal minimum feature sizes continue to decrease, control of the variability of these feature sizes becomes more critical. For example, the sensitivity of given critical dimensions of patterned features to exposure tool and mask manufacturing imperfections as well as resist and thin film process variability is becoming more significant. In order to continue to develop manufacturable processes in light of the limited ability to reduce the variability of exposure tool and mask manufacturing parameters, it is desirable to reduce the sensitivity of critical dimensions of patterned features to these parameters.
As feature sizes decrease, semiconductor devices are typically less expensive to manufacture and have higher performance. In order to produce smaller feature sizes, an exposure tool having adequate resolution and depth of focus at least as deep as the thickness of the photoresist layer is desired. For exposure tools that use conventional or oblique illumination, better resolution can be achieved by lowering the wavelength of the exposing radiation or by increasing the numerical aperture of the lithographic exposure apparatus.
The skilled artisan will appreciate that the resolution varies in proportion to the exposure wavelength and varies in inverse proportion to the numerical aperture (NA) of the projection optical system. The NA is a measure of a lens' capability to collect diffracted radiation from a mask and project it onto the wafer. The resolution limit R (nm) in a photolithography technique using a reduction exposure method is described by the following equation:R=k1λ/(NA)where:                λ is the wavelength (nm) of the exposure radiation;        NA is the numerical aperture of the lens; and        k1 is a constant dependent inter alia on the type of resist used.        
It follows that one way to increase the resolution limit is to increase the numerical aperture (high NA). This method, however, has drawbacks due to an attendant decrease in the depth of focus, difficulty in the design of lenses, and complexity in the lens fabrication technology itself. An alternative approach is to decrease the wavelength of the exposure radiation in order to form finer patterns, e.g. to support an increase in the integration density of LSI (Large Scale Integration) devices. For example, a 1-Gbit DRAM requires a 0.2-micrometer pattern while a 4-Gbit DRAM requires a 0.1-micrometer pattern. In order to realize these patterns, exposure radiation having shorter wavelength can be used.
However, because of increased semiconductor device complexity that results in increased pattern complexity, and increased pattern packing density on a mask, distance between any two opaque mask areas has decreased. By decreasing the distances between the opaque areas, small apertures are formed which diffract the radiation that passes through the apertures. The diffracted radiation results in effects that tend to spread or to bend the radiation as it passes, so that the space between the two opaque areas is not resolved; in this way, diffraction is a severe limiting factor for optical photolithography.
A conventional method of dealing with diffraction effects in optical photolithography concerns the use of a phase shift mask, which replaces the previously discussed mask. Generally, with radiation being thought of as a wave, phase shifting is a change in timing (phase) of a regular sinusoidal pattern of radiation waves that propagate through a transparent material. Although the rest of this discussion will generally concentrate on transmissive phase shift masks, it should be realized that reflective phase shift masks can also be contemplated (e.g. for use with the wavelengths associated with EUV radiation). The current invention encompasses both these concepts.
Typically, phase-shifting is achieved by passing radiation through areas of a transparent material of either differing thickness or through materials with different refractive indexes, or both, thereby changing the phase or the periodic pattern of the radiation wave. Phase shift masks reduce diffraction effects by combining both diffracted radiation and phase-shifted diffracted radiation so that constructive and destructive interference takes place favorably. On the average, a minimum width of a pattern resolved by using a phase shifting mask is about half the width of a pattern resolved by using an ordinary mask.
There are several different types of phase shift structures. These types include:
alternating aperture phase shift structures, sub-resolution phase shift structures, rim phase shift structures, and chromeless phase shift structures. “Alternating Phase Shifting” is a spatial frequency reduction concept characterized by a pattern of features alternately covered by a phase shifting layer. “Sub-resolution Phase Shifting” promotes edge intensity cut-off by placing a sub-resolution feature adjacent to a primary feature and covering it with a phase shifting layer. “Rim Phase Shifting” overhangs a phase shifter over a chrome mask pattern.
In the case of transmissive masks, these phase shift structures are generally constructed in masks having three distinct layers of material. An opaque layer is patterned to form blocking areas that allow none of the exposure radiation to pass through. A transparent layer, typically the substrate plate (e.g. of quartz or calcium fluoride), is patterned with transmissive areas, which allow close to 100% of the exposure radiation to pass through. A phase shift layer is patterned with phase shift areas which allow close to 100% of the exposure radiation to pass through, but phase-shifted by 180° (π). The transmissive and phase-shifting areas are situated such that exposure radiation diffracted through each area is canceled out in a darkened area therebetween. This creates the pattern of dark and bright areas, which can be used to clearly delineate features. These features are typically defined by the opaque layer (i.e. opaque features) or by openings in the opaque layer (i.e. clear features).
For semiconductor (and other device) manufacture, alternating aperture phase shift masks may typically be used where there are a number of pairs of closely packed opaque features. However, in situations where a feature is too far away from an adjacent feature to provide phase shifting, sub-resolution phase shift structures typically may be employed. Sub-resolution phase shift structures typically may be used for isolated features such as contact holes and line openings, wherein the phase shift structures may include assist-slots or outrigger structures on the sides of a feature. Sub-resolution phase shift structures are below the resolution limit of the lithographic system and therefore do not print on the substrate. One shortcoming of sub-resolution phase shift structures is that they require a relatively large amount of real estate on the mask.
Rim phase shifting masks include phase shift structures that are formed at the rim of features defined by opaque areas of the mask. One problem with rim phase shift structures is that they are difficult to manufacture. In the case of rim phase shift structures, multiple lithographic steps must be used to uncover the opaque layer so that it can be etched away in the area of the rim phase shifter. This step is difficult, as the resist used in the lithographic step covers not only the opaque layer but also trenches etched into the substrate.
In general, improvement of the integration density of semiconductor integrated circuits in recent years has been achieved mainly through a reduction in size of the various circuit patterns. These circuit patterns are presently formed mainly by lithography processes using a wafer stepper or step-and-scan apparatus.
FIG. 1 shows the structure of such a prior art lithographic apparatus. Mask 108 is illuminated by the radiation emitted from illumination system 102. An image of mask 108 is projected onto a photoresist film coated on wafer 120, which is the substrate to be exposed through projection system 110. As shown in FIG. 1, illumination system 102 includes a source 100, condenser lens 104, and aperture 106 for specifying the shape and size of the effective source. Projection system 110 includes a projection lens 112, pupil filter 114, and aperture 116 arranged in or near the pupil plane of focussing lens 118 to set the numerical aperture (NA) of the lens.
As discussed earlier, the minimum features size R of patterns transferable by an optical system is approximately proportional to the wavelength λ of the radiation used for exposure and inversely proportional to the numerical aperture (NA) of the projection optical system. Therefore, R is expressed as R=k1 λ/NA, where k1 is an empirical constant and k1=0.61 is referred to as the Rayleigh limit.
In general, when the pattern dimensions approach the Rayleigh limit, the projected image is no longer a faithfull reproduction of the mask pattern shape. This phenomenon is caused by so-called optical proximity effects (OPEs) and results in corner rounding, line-end shortening, and line width errors, among other things. To solve this problem, algorithms have been proposed that can be used to predistort the mask pattern so that the shape of a projected image takes on the desired shape.
Moreover, approaches have been described which improve the resolution limit of a given optical system, resulting effectively in a decreased value of k1. Adoption of a phase shifting mask, such as described above, is a typical example of this approach. A phase shifting mask is used to provide a phase difference between adjacent apertures of a conventional mask.
A chromeless phase shifting mask method is known as a phase shifting method suitable for the transfer of a fine isolated opaque line pattern, which is needed, for example, for the gate pattern of a logic LSI device.
Off-axis illumination and pupil filtering are methods additionally known for improving images. According to the off-axis illumination method, the transmittance of aperture 106 is modified in the illumination system 102 of FIG. 1 (prior art). One particular embodiment of this method changes the illumination intensity profile so that the transmittance at the margin becomes larger than that of the central portion, which is particularly effective to improve the resolution of a periodic pattern, as well as the depth of focus. The pupil filtering method is a method of performing exposure through a filter (pupil filter) located at the pupil position of a projection lens to locally change the amplitude and/or phase of the transmitted radiation. For example, this approach makes it possible to greatly increase the depth of focus of an isolated pattern. Furthermore, it is well known that the resolution of a periodic pattern can further be improved by combining the off-axis illumination method and the pupil filtering method.
Nonetheless, an inherent problem with a conventional transmission mask, such as the ones described above, is that the mask substrate (plate) generally undergoes a decrease in transmissivity as the wavelength of radiation emitted from an exposure radiation source is decreased so as to obtain finer patterns. For example, a quartz material substrate becomes more opaque as the wavelength of the radiation source decreases, particularly when the wavelength is less than 200 nm. This decrease in transmissivity affects the ability to obtain finer resolution patterns. For this reason, a material for a transmission phase shifting mask that can obtain a high transmissivity with respect to radiation having a short wavelength is needed. It is, however, difficult to find or manufacture such a material having a high transmissivity with respect to short-wavelength exposure radiation.
An example of a photomask pattern is shown in FIG. 2 (prior art). Passage of radiation around the illustrated features causes diffraction of the radiation into discrete dark and bright areas. The bright areas are known as the diffraction orders and the collective pattern they form is mathematically describable by taking the Fourier transform of the collective opaque and transparent regions. The pattern that is observed in its simplest personification has an intense diffraction order, called the 0th order, surrounded in a symmetrical fashion by less intense diffraction orders. These less intense orders are called the plus/minus first (±1st) order; plus/minus second (±2nd) order; and so on into an infinity of orders. For the same feature width, different diffraction patterns are formed for dense and isolated features. FIG. 3(A) (prior art) shows the magnitudes of relative electric fields and respectively pupil positions (X) of diffraction orders for a dense feature, while FIG. 3(B) (prior art) shows the magnitudes of diffraction orders for an isolated one. The center peak observed in each plot is the 0th order.
The 0th order contains no information about the pattern from which it arose. The information about the pattern is contained in the non-zero orders. However, the 0th order is spatially coherent with the higher orders so that, when the beams are redirected to a point of focus, they interfere, and in doing so construct an image of the original pattern of opaque and transparent objects. If all the diffraction orders are collected, a perfect representation of the starting object is obtained. However, in high-resolution lithography of small-pitch features, where pitch is the sum of the width of the opaque and transparent objects, only the 0th and the ±1st orders are collected by the projection lens to form the image. This is because higher orders are diffracted at higher angles that fall outside of the lens pupil as defined by the numerical aperture (NA).
As depicted in FIG. 4(A) (prior art), the 0th order 402 and the ±1st orders 404 lie within the lens pupil 406. As further depicted in FIG. 4(A), the ±2nd orders 408, lie outside the lens pupil 406. Further, as seen in FIG. 4(B) (prior art), a corresponding aerial image is formed during exposure (I indicates intensity, and H indicates horizontal position). The photoresist pattern is then delineated from this aerial image.
It has long been known that it is only necessary to collect two diffraction orders, such as either with the 0th order and at least one of the higher diffraction orders, or simply two higher orders without the 0th order, to form the image.
As depicted in FIG. 5(A) (prior art), radiation transmitted through a focussing lens 502 is represented by that which is normal 504 to the object (not shown), and that which transmits through the edges 506, 508 of the focussing lens 502. Although radiation is continuously transmitted throughout the entire surface of lens 502, the three radiation paths 504-508 are represented to illustrate phase matching of different radiation paths. At point 510, the three radiation paths 504-508 focus and are in phase together. When the three radiation paths 504, 512, and 514 focus together at point 516, however, they are not in phase.
The phase error from a change in path-lengths of 512 and 514 from respective path-lengths 506 and 508 results in a finite depth of focus, DoF, of the system.
One may improve the tolerance to variations in relative phase error caused by aberrations like defocus as depicted in FIG. 5(A). FIG. 5(B) (prior art) represents how, by eliminating the radiation path that is normal to the object, variations to the phase error may be reduced. Again, although radiation is continuously transmitted throughout the surface of lens 502, the two radiation paths 506 and 508 are represented to illustrate phase matching of different radiation paths. At point 510, the two radiation paths 506 and 508 focus and are in phase together. When the two radiation paths 512 and 514 focus together at point 516, they are in phase. Without the radiation path 504 as seen in FIG. 5(A), the phase error from the increased path-lengths of 512 and 514 over respective path-lengths 506 and 508 is eliminated, resulting in an infinite depth of focus, DoF, of the system. Eliminating the radiation path normal to the object may be accomplished by placing an obscuration in the center of the radiation source, thus eliminating radiation normal to the object and allowing only oblique illumination, as depicted for example in FIG. 6(A).
FIG. 6(A) (prior art) depicts a lithographic “on-axis” projection system (“C” indicates conventional) wherein the illumination configuration 602 is such as to permit transmission of radiation normal to the object. In the figure, radiation passes through the reticle, comprising a quartz substrate 604 and chrome patterns 606, through the lens aperture 608, into lens 610, and is focused into area 612. FIG. 6(B) (prior art) depicts exemplary lithographic “off-axis” projection systems wherein an annular (A) illumination configuration 614, or quadrupole (Q) illumination configuration 616, prohibits transmission of radiation normal to the object. In the figure, radiation passes through the vitreous substrate 604, past the chrome patterns 606, through the lens aperture 608, into lens 610, and is focused into area 618. Comparing FIGS. 6(A) and 6(B), it is noted that the Depth of Focus (DoF) of FIG. 6(A) is smaller than that of FIG. 6(B).
Lowering the 0th order's magnitude to be the same as or less than that of the 1st order improves the imaging tolerance of this two-beam imaging system. One method for tuning the magnitude of the diffraction orders is to use weak phase shift masks. Strong phase shift masks and weak phase shift masks differ in operation and effect.
Strong phase shift masks eliminate the zero-diffraction order and double the resolution through a technique of frequency doubling. To understand how strong phase shifters work, it is useful to think of the critical pitch as having alternating clear areas adjacent to the main opaque feature. Because of the alternating phase regions, the pitch between same-phase regions is doubled. This doubling halves the position at which the diffraction orders would otherwise pass through the projection lens relative to the critical pitch, thus making it possible to image features with half the pitch allowed by conventional imaging. When the two opposing phase regions add through destructive interference, to build the final image, their respective zero-order radiation is equal in magnitude but of opposite phase, thus canceling. Imagining is done only with the frequency-doubled higher orders. On the other hand, weak phase shift masks dampen the zero-order radiation and enhance the higher orders. Weak phase shift masks from their phase shift between adjacent features by creating electric fields of unequal magnitude and of opposite phase, with the field immediately adjacent to a critical feature having the lesser of the magnitudes. The net electric field reduces the magnitude of the zero order while maintaining the appropriate phase.
Weak phase shift masks permit an amount of exposure radiation to pass through objects in a fashion that creates a difference in phase between coherently linked points while having an imbalance in the electric field between the shifted regions. FIG. 7(A) (prior art) depicts a substrate 702 and a mask pattern 704 that does not permit phase shifting. FIG. 7(C) (prior art) is a graph illustrating how the 0th order's magnitude is larger than that of the ±1st orders' magnitude from a non-phase shifting mask as depicted in FIG. 7(A). FIG. 7(B) (prior art) depicts a substrate 702 and a mask pattern 706 that permit phase shifting (in the Figure, Φ is phase, t is thickness, n is index of refraction and λ is wavelength). FIG. 7(D) (prior art) is a graph illustrating how the 0th order's magnitude is decreased to be comparable to that of the ±1st orders' magnitude from a phase shifting mask as depicted in FIG. 7(B).
Several types of phase-shifting masks are known in the art, such as the rim, attenuated or embedded (or incorrectly halftone), and unattenuated or chromeless (or transparent) shifter-shutter phase-shifting masks.
FIG. 8(A) (prior art) is a cross-sectional view of a rim phase-shifting mask 802, comprising radiation transmitting portions 804, and radiation inhibiting portions 806. FIG. 8(B) (prior art) is a graph representing the amplitude (E) of the E-field at the mask, whereas FIG. 8(C) (prior art) is a diagram representing the magnitude of the 0th diffraction order 810, and ±1st orders 812, 814, resulting from use of the mask depicted in FIG. 8(A).
FIG. 9(A) (prior art) is a cross-sectional view of an attenuated or embedded phase-shifting mask 902 having an attenuation of 5%, comprising a radiation attenuating portion 904. FIG. 9(B) (prior art) is a graph representing the amplitude of the E-field at the mask, whereas FIG. 9(C) (prior art) is a diagram representing the magnitude of the 0th diffraction order, and ±1st diffraction orders resulting from use of the mask depicted in FIG. 9(A). FIG. 9(D) (prior art) is a cross-sectional view of an attenuated or embedded phase-shifting mask 912 having an attenuation of 10%, comprising a radiation attenuating portion 914. FIG. 9(E) (prior art) is a graph representing the amplitude of the E-field at the mask, whereas FIG. 9(F) (prior art) is a diagram representing the magnitude of the 0th diffraction order, and ±1st diffraction orders resulting from use of the mask depicted in FIG. 9(D).
FIG. 10(A) (prior art) is a cross-sectional view of an unattenuated or chromeless (or transparent) shifter-shutter phase-shifting mask 1002, comprising a radiation-shifting portion 1004. FIG. 10(B) (prior art) is a graph representing the amplitude of the E-field at the mask, whereas FIG. 10(C) (prior art) is a diagram representing the magnitude of the 0th diffraction order 1006, and ±1st diffraction orders 1008, 1010 resulting from use of the mask depicted in FIG. 10(A).
Typically, the phase-shifting masks of FIG. 8 through FIG. 10 form their phase-shift differently, but, relative to their non-phase-shifted counterpart, they all yield a 0th diffraction order of smaller amplitude and a first diffraction order of larger amplitude, as regards electric field. Which ratio of 1st to 0th diffraction order magnitude is optimal depends on the pitch of the feature being imaged, along with the shape of the illumination configuration and the desired printing size in the developed photoresist. These tuned diffraction patterns are then used with off-axis illumination to image smaller pitches with better tolerance to imaging process variation.
The concept of manipulation of the amplitude ratio of 0th to 1st diffraction orders has conventionally been restricted to using certain weak phase-shifting techniques with biasing features and sub-resolution assist features.
FIG. 11(A) (prior art) depicts a conventional biasing technique used to resolve a desired feature. As seen in FIG. 11(A), biasing (B) bars 1102 and 1104 are situated adjacent the mask of the primary feature 1106. FIG. 11(B) depicts a half-tone biasing (HB) technique known to the applicants of the instant application and described in U.S. Pat. No. 6,114, 071 (incorporated herein by reference), used to resolve a desired feature. As seen in FIG. 11(B), half-tone biasing bars 1108 and 1110 are situated adjacent the mask of the desired feature 1112. FIG. 12 (prior art) depicts a conventional photoresist mask 1202. The photoresist mask 1202 comprises a plurality of scatter bars 1204, serifs 1206, and chrome shields 1208.
For conventional attenuated phase shifters, transparency of the shifter materials typically may be adjusted, and used along with biasing and sub-resolution assist features. Transparency of the shifters typically ranges from 3% to 10%, wherein higher transmissions such as from 10% to 100% are reported to be optimal for pitches where the space between the features is larger than the phase-shifted line. FIG. 13 (prior art) shows the dependence of image contrast, as defined by the Normalized Image Log Slope (NILS), with respect to varying transmittance (T) of its phase-shifted material for a 175 nm line on a 525 nm pitch (FIG. 13A) and a 1050 nm pitch (FIG. 13B). Each curve in the figure represents a different focus (F) setting. The curve with the largest NILS is the most focussed, and has an F-value of zero; further, with each change in focus, the NILS of each respective curve decreases. FIG. 13A shows that the best transmission for the 175 nm line with the 525 nm pitch structure is 0.35 to 0.45. FIG. 13B shows that the best transmission for the 175 nm line with the 1050 nm pitch structure is 0.25 to 0.35.
An example of a 100% transparent attenuated phase-shifting technology is the previously mentioned, chromeless shifter-shutter, such as depicted in FIG. 10. Using a chromeless shifter-shutter, phase-edges of a pattern typically may be placed within an area that is 0.2 to 0.3 times the exposing wavelength λ divided by the numerical aperture NA of the projection lens. For lines larger or smaller than this, the destructive interference is insufficient to prevent exposure in an area not be exposed. Printing features larger than this is accomplished in one of two ways. The first places an opaque layer in the region that is to stay dark with the feature edges being opaque or rim-shifted (FIG. 14; prior art). The second, as depicted in FIG. 15 (prior art), creates a dark grating 1502 by placing a series of features 1504 whose size meets the criteria for printing an opaque line 1506 using chromeless technology. In FIGS. 14 and 15, “IM” denotes image, “CPSM” denotes a chromeless phase shift mask, “OP” indicates opaque and “PS” denotes phase shift.
Conventionally, chromeless phase shifting masks have not worked with off-axis exposure as the shifter (feature) sizes and shutter (space) sizes approach one another. FIGS. 16(A) through 16(C) depict a conventional chromeless phase shifting mask. In FIG. 16(A) (prior art), 1602 is a cross-sectional view of a portion of a conventional chromeless phase shifting mask, comprising shifters 1604, and shutters 1606, wherein the shifter length is substantially equal to the shutter length. FIG. 16(B) (prior art) is a graph representing the amplitude of the E-field at the mask 1602. FIG. 16(C) (prior art) is a diagram representing the magnitudes of the ±1st diffraction orders 1608 and 1601 for the mask of FIG. 16(A). As seen in FIG. 16(C) there is no 0th diffraction order. The functional limit of the relative sizes of the shifter and shutters of conventional chromeless phase shifting masks results from the integrated electric fields of the two opposing phase-shifted regions being equal. This balanced condition cancels the 0th diffraction order, making it impossible to get the prerequisite 0th diffraction order needed for using off-axis illumination.
To summarize, each of the above-described, conventional, weak phase-shifting techniques solves certain imaging problems. However, each technique has accompanying drawbacks. For example, the rim, attenuated or embedded, and unattenuated or chromeless (or transparent) shifter-shutter phase-shifting masks provide large ratios of the 0th to ±1st diffraction orders. Prior-art attempts to manipulate these ratios include using biasing techniques coupled with an attenuated phase shifting mask. However, these prior art attempts include complex manufacturing steps and yield inefficient masks as a result of the attenuation. Furthermore, unattenuated shifter-shutter phase-shifting mask additionally fail to yield accurate images with off-axis illumination as the shifter and shutter sizes approached one another.